Any of those changes may affect your rankings but changing the website is necessary unless you want to stay where you are. So the best advice I can give is for you to keep experimenting and take note of what change caused what. Be aware though that each change takes weeks and months, not days, so be ready to invest some time into learning SEO.

You will definitely need to 301 the old URLs to the new ones. If you don't then you will very likely see a drop in rankings! WordPress often takes care of this for you (if that is your CMS). Use Google to find a redirect checker to make sure all the redirects function properly.

If you don't know what the old URLs are then maybe you should check out Google's index (just type "site:yourwebsite.com") and check out the cached version of your pages. It should give you a number of your old URLs. It depends how recently the big G crawled your links though.
You may also have some luck with the way back machine (http://archive.org/web/).

I would redirect what you know. And anything else that changed go one by one to change your links. Ranking will drop as I saw that happen to me. But in the long run it made more sense for changing the link structure on my site.

OMg. I spent money on this site. Anyway I could not find traces of my old links because as soon as I was done and submitted my site map. OMG. So what I can do with my old articles as well as new ones? I have about 42 articles in total and none of them are less than 500 words.

When I changed my permalinks from website/category/blog to website/blog I used a 301 redirect plugin and manually updated each post to the correct link. I had about 150 posts at the time. Took about an hour of cut and pasting. Should be easy enough to figure out the original structure by looking at an archive of the website. Not sure if there is an easier way to do this, but it made sure everything went to the new correct link.

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